1. Field of the Invention
The invention relates to a technique for jointly performing bit synchronization and error detection of received digital data bursts in a time division multiplexed/time division multiple access (TDM/TDMA) system, such as that which will be used in conjunction with low power portable digital telephony.
2. Description of the Prior Art
People by their very nature are highly mobile; no where is this more true than in modern day society with its myriad forms of travel. At the same time, many people increasingly have a need to be able to telephonically communicate with others particularly while they are on "the go", i.e. while they are moving.
However, this need for mobile communications, which existed for quite some time, has remained basically unsatisfied. Since telephones traditionally have cords, any movement of the telephone was traditionally limited by the length of its cord. For many years, only a veritable handful of telephones actually traveled with their users. These mobile telephones included aeronautical, marine and other forms of early radio telephones. Inasmuch as these mobile telephones were priced well beyond the affordability of the average telephone subscriber, none of these radio telephones ever encountered widespread use. Accordingly, for the vast majority of subscribers, a telephone set was installed at each subscriber location and there it remained unless it was re-installed elsewhere. Thus, these subscribers either remained close to their telephone and thus restricted their mobility particularly in the anticipation of receiving a telephone call, or intentionally sought out a public or private telephone located along their route of travel whenever the need arose to place a telephone call.
Now with increasing sophistication of miniaturized electronic technology and decreasing attendant cost thereof, various vendors provide a number of devices (and/or services) that offer tetherless telephony. These devices, explained in more detail below, attempt to free a subscriber from being bound by the ambulatory constraints imposed by existing wireline telephone sets. In effect, each of these devices now permits subscribers effectively, at least within a certain extent, to take their telephone with them, obtain exchange access, and remain in communication wherever they go. These devices include cordless telephones, cellular mobile radio transceivers, public packet radio data network transceivers and radio pagers. As a growing number of consumers perceived the freedom of movement offered by these devices, a large demand was created for these devices. Moreover and not unexpectedly, as the prices of these devices continue to fall due to manufacturing economies and technical developments, the demand for these devices correspondingly continues to substantially increase. Specifically, approximately 25 million cordless telephone sets are in use today throughout the United States with demand for these sets continuing to rise as the price of cordless telephones with increasing sophisticated has remained within a $100.00 to $200.00 range. In addition, approximately three million cellular telephone sets are currently in use throughout the United States. As the price of various cellular sets falls from more than a $1000.00 which occurred merely a year ago to only a few hundred dollars today, the demand for these sets has increased precipitously. As a result, the number of installed sets has climbed at such an astonishing rate that in certain urban areas, such as New York, the number of sets in use at peak times is beginning to strain the capacity of the existing cellular network to handle the concomitant call traffic.
While each of the present tetherless telephonic technologies possesses certain advantages, each technology also unfortunately has certain drawbacks that significantly restrict its use. In this regard, see, e.g., Cox, "Portable Digital Radio Communications--An Approach to Tetherless Access", IEEE Communications Magazine, Vol. 27, No. 7, July 1989 pages 30-40; and Cox, "Universal Digital Portable Radio Communications", Proceedings of the IEEE, Vol. 75, No. 4, April 1987, pages 436-476.
Specifically, as to cordless telephones, such a telephone consists of two transceivers: a base unit and a handset, that collectively form a low power duplex analog radio link. The base unit is connected, typically by a subscriber to a wireline access point in a conventional telephone network in lieu of or as a replacement for a wireline telephone, in order to implement a tetherless substitute for a telephone cord. Once connected, the base unit appears to the telephone network as a conventional telephone. The base unit contains a transmitter and a receiver, and simple control and interface apparatus for dialing, accepting ringing, terminating calls and coupling voice from the telephone line to the transmitter and from the receiver within the base unit to the telephone line. The handset, which is truly portable, contains simple control logic for initiating, receiving and terminating calls with the base unit and for turning its own transmitter on and off. To provide true duplex operation, separate carrier frequencies are used by the transmitters in the base unit and handset. Since cordless telephones operate with very low input power to their transmitter, usually on the order of only several milliwatts, the handset generally utilizes several small rechargeable batteries as its power source. This enables the handset to be made relatively small, lightweight and to be continuously used for a relatively long period, typically several hours, before its batteries require recharging. Furthermore, the very low level of power radiated from the handset poses essentially no biological radiation hazard to its user.
Unfortunately, the primary disadvantage of cordless telephones is their highly limited service area. Because cordless telephones use relatively low transmitter power, these telephones have a maximum range that varies from typically a few hundred to a thousand feet, which in turn results in a very small service area. A secondary disadvantage associated with cordless telephones stems from the limited number of available frequencies. At present, only a few separate frequencies, typically up to 10 duplex channels, have been allocated by the Federal Communications Commission (FCC) for use by cordless telephones. Moreover, early cordless telephones by their very design have been very susceptible to co-channel interference. This interference arises by the simultaneous operation of two or more cordless telephones situated in close proximity to each other, such as in an immediate neighborhood of a residential area. In a very small geographic area with a very low density of users, a reasonable probability exists that within this area one or more duplex pairs will not be in use at any one time, and, as such, this interference will not occur therein. Nevertheless, in an effort to avoid this interference, relatively sophisticated cordless telephones are now capable of operating on any one of a number of pre-programmed duplex pairs with either the user or the telephone itself selecting, manually in the case of the user and automatically by the telephone, the specific pair that is to be used at any one time. Unfortunately, if a sufficient number of cordless telephones are in use in a very densely populated area, such as an apartment building, pair selection may not be sufficient to eliminate the expected incidences of co-channel interference that results from undisciplined and uncoordinated duplex pair assignment and the resulting chaos experienced by users situated therein. In addition, since cordless telephones rely on analog modulation of a duplex pair, conversations occurring over a cordless telephone are highly vulnerable to eavesdropping. Furthermore, a cordless telephone only provides limited protection against unauthorized long distance or message units calls being made therethrough. While pre-programmed digital or tone access codes are being used between individual handset-base unit pairs and provide sufficient protection against casual attempts at unauthorized access, these codes are not sufficiently sophisticated to successfully deter a determined orderly assault on a cordless telephone by an unauthorized user. Furthermore, while cordless telephones provide limited portable radio access to a wireline access point, from a network standpoint cordless telephones do not eliminate the need for telephone lines, i.e. a customer drop, to be run to each subscriber.
Nonetheless, in spite of these severe service restrictions, cordless telephones are immensely popular for the freedom, though very limited, that they furnish to their users.
In contrast to the very limited range provided by cordless telephones, cellular mobile radio systems accommodate wide ranging vehicular subscribers that move at relatively high speeds. These systems utilize a relatively high power 850 MHz transmitter, typically operating at an input of approximately 0.5 watt to several tens of watts, in a mobile unit with a relatively high efficiency antenna to access a wireline telephone network through a fixed cell-site (base station). The base station also uses a high power transmitter in conjunction with a tall antenna, typically erected on a tower or tall building, to provide a relatively large coverage area. Due to the expense, typically ranging to $300,000 exclusive of land and building costs, and the antenna size associated with each base station, the least number of base stations is often used to cover a given area. Nonetheless, this arrangement generally provides a circular service area centered on a base station with a radius of approximately 5-10 miles therefrom. In use, a cellular radio system that covers a large region often encompassing a city, its suburbs and major access highways typically includes a number of geographically dispersed base stations. The base stations, containing radio receivers and transmitters and interface and control electronics, are connected by trunks to, and coordinated and controlled by one or more Mobile Telephone Switching Offices (MTSOs) that, in turn, also provide access to the conventional wireline telephone network. All of the duplex radio channels available to the entire system are sub-divided into sets of channels. The radio equipment in each base station has the capability of using channels from one of the channel sets. These sets are allocated to the base station in a pattern that maximizes the distance between base stations that use the same sets so as to minimize average co-channel interference occurring throughout a service region. One or more channels are designated for initial coordination with the mobile sets during call setup.
Each mobile (or hand-held) cellular transceiver used in the system contains a receiver and a transmitter capable of operating on any duplex radio channel available to the cellular system. Calls can be made to or from any mobile set anywhere within the large region covered by a group of base stations. The control electronics in the mobile transceiver coordinates with a base station on a special call setup channel, identifies itself, and thereafter tunes to a channel designated by the base station for use during a particular call. Each duplex channel uses one frequency for transmission from base-to-mobile and a different frequency for transmission from mobile-to-base. The signal strength of calls in progress is monitored by the base stations that can serve those calls. Specifically, when the signal strength for a given call drops below a pre-determined threshold, typically due to movement of the cellular subscriber from one cell to another, the MTSO connected to that base station coordinates additional signal strength measurements from other base stations which surround the station that is currently handling the call. The MTSO then attempts to switch ("handoff") the call to another duplex channel if one of the other base stations is receiving a stronger signal than that being received at the base station that is currently handling the call. This handoff of calls, totally transparent to the cellular subscriber, preserves the quality of the radio circuit as the subscriber moves throughout the service region. Moreover, calls are handed off from one MTSO to another, as the subscriber transits from one service area into another. Inasmuch as frequency usage is coordinated, relatively efficient use is made of the available frequency spectrum while minimizing the likelihood co-channel interference. In each different geographic service area within the United States, there are two competing cellular systems using different frequencies.
Though cellular mobile radio systems provide wide range, these systems suffer various drawbacks. First, cellular systems were originally designed for use in motor vehicles whose electrical systems could readily provide sufficient power. While portable hand-held cellular transceivers do exist, they must operate with sufficient transmitter input power, typically at least 0.5 watt, to reliably reach a base station. This, in turn, requires that a relatively large battery must be used within the portable cellular transceiver. However, due to the limits of present rechargeable battery technology, the amount of time that the portable transceiver can be used before it requires recharging is often quite limited. Furthermore, the cost of these rechargeable batteries and hence of the portable transceiver is rather high. Moreover, high radiated power levels, such as that which emanate from a mobile or portable cellular transceiver, may be sufficient to pose a potential biological radiation hazard to its user. Furthermore, since cellular systems were not designed to compensate for radio attenuation occurring within buildings, these systems are only able to provide little, if any, service within a building. Low power portable cellular transceivers are neither operationally compatible with large cell sizes nor designed to match the needs of fast moving vehicular user and thus often provide poor communication in many areas within these cells. In addition, since cellular systems rely on merely frequency modulating a carrier with voice or data, these systems are also susceptible to eavesdropping. Lastly, from a network perspective, cellular systems are quite inefficient. Due to the inclusion of MTSOs with trunks connected to individual base stations, backhaul of cellular traffic, over wired trunks, often occurs over several miles prior to its entrance into the wireline network, thereby resulting in a wasteful overbuild of network transport facilities.
Public packet radio data networks presently exist to handle infrequent bursts of digital data between a fixed base station and a number of portable data transceivers. The fixed site has a transmitter that uses several tens of watts; while each portable data transceiver uses a transmitter that operates at a level of several watts. As such, reliable coverage is provided over a service area that may extend several miles in radius from a base station. Individual base stations are connected by a fixed distribution facility to a controller that can, in turn, be connected to either a local exchange network, to handle voice-band data, or a packet-data network which itself interconnects various computers. Multiple users contend for transmission time on typically a single radio channel. Data transmissions on the channel are set up in either direction through bursts of coordinating data,i.e. handshaking, that occur between a base station and a portable data transceiver. Appropriate controller and radio link protocols are used to avoid packet collisions. Once a data transfer is complete between that base station and a data transceiver, the channel is immediately available for re-use by others. Although data bursts are transmitted at relatively high power, each burst is transmitted for only a short duration. As such, the average power consumption for a portable data transceiver is far less than that associated with a portable cellular transceiver thereby allowing physically smaller internal batteries to be used with portable data transceivers than those used in portable cellular transceivers. Nevertheless, the high radiated power levels associated with a portable data transceiver again pose a potential biological radiation hazard to its user. In addition, these networks disadvantageously suffer from limited digital transmission capacity which restricts these networks to carrying short data bursts and not voice, and, like cellular systems, experience coverage restraints when used within buildings.
In contrast to the tetherless systems discussed above, radio paging systems provide simple unidirectional transmission from a fixed location to a specifically addressed portable pager, which when received provides an alerting tone and/or a simple text message. Paging systems provide optimized one-way communication over a large region through a high power transmitter, typically a few kilowatts, that uses high antennas at multiple sites to provide reliable coverage throughout the region. Satellite based paging systems are also in operation to provide extended service regions. Since a pager is merely a receiver with a small annunciator, its power requirement is very low. As such, a pager is quite small, light weight, reliable, relatively low cost, and can operate for long intervals before its batteries need to be recharged or replaced.
Due to the advantages in size, cost and operating duration offered by pocket pagers, attempts exist in the art, to impart limited two-way communication into paging systems which are themselves highly optimized for one-way traffic. One such attempt includes incorporation of an "answer back" message through "reverse" transmission links between the individual pagers and the fixed sites. While these attempts have met with great difficulty, these attempts nevertheless indicate that a substantial demand exists for an inexpensive two-way portable truly tetherless telephonic service that overcomes the range limitations associated with cordless telephones and the weight and cost limitations associated with portable cellular systems.
Furthermore, various intelligent network services are now being offered by the local telephone operating companies in an attempt to provide wireline subscribers with a certain degree of call mobility when they are away from their own wireline telephones. These services include call transfer and call forwarding. Both call transfer and call forwarding allow a subscriber to program a local switch, using any pushbutton telephone, to transfer all subsequently occurring incoming calls that would otherwise be routed to this subscriber's telephone to a telephone associated with a different wireline telephone number that the subscriber desires anywhere in the world either for a given period of time, as in call transfer, or until that subscriber appropriately re-programs the switch with a different forwarding number, as in call forwarding. In this manner, the subscriber can, to a certain extent, continually instruct the telephone network to follow his or her movements and thereby route his or her incoming calls to a different number in unison with that subscriber's actual route of travel. Unfortunately, with these services, the subscriber must manually interact with the network and continually enter a new forwarding telephone number(s) coincident with his or her continuing travel such that the network is always cognizant of the current telephone number to which his calls are to be forwarded.
Thus, a substantial overall need exists in the art for a truly portable personal communication technology that is designed for pedestrian use and which utilizes small, lightweight and relatively inexpensive portable transceivers while eliminating, or at least substantially reducing, the performance drawbacks associated with the use of currently existing tetherless telephonic technologies in portable communication applications.
In an attempt to provide this needed technology, the art has turned to low power portable digital telephony. In essence, this technology, similar to cellular radio, uses a fixed base unit (hereinafter referred to as a port) and a number of mobile transceivers (hereinafter referred to as portables) that can simultaneously access that port on a multiplexed basis. However, in contrast to cellular radio, portable digital telephony uses low power multiplexed radio links that operate on a time division multiplexed/time division multiple access (TDM/TDMA) basis to provide a number of separate fully duplex demand-assigned digital channels between a port and each of its associated portables. Specifically, each port would transmit time division multiplexed (TDM) bit streams on a pre-defined carrier frequency, with, in turn, each portable that accesses that port responding by transmitting a TDMA burst on a common, though different, pre-defined carrier frequency from that used by the port. Quadrature phase shift keying (QPSK), with an inter-carrier spacing of 150 to 300 KHz and within a given operating frequency band situated somewhere between approximately 0.5 to 5 GHz would be used by both the port and portables. The power used by the transmitter in the portable would range between 5-10 milliwatts or less on average and provide a range of several hundred to a thousand feet. As such, the resulting low radiated power would pose essentially no biological radiation hazard to any user. In addition, the port antenna would be relatively small and suitable for mounting on a utility or light pole. With this transmission range, a port could simultaneously serve typically 20-30 separate locally situated portables. The same TDM channels would be re-used at ports that are spaced sufficiently far apart to reduce co-channel interference to an acceptably low level but yet conserve valuable spectrum To provide access to the wireline telephone network, each port would be interfaced, typically through a conventional fixed distribution facility, over either a copper or fiber connection to a switching machine at a local central office. The switching machine would be suitably programmed, in a similar manner as is an MTSO, to controllably and automatically handoff calls from one port to another as subscribers move their portables from port to port.
Due to the very limited transmitter power, each portable is anticipated to be very light-weight, physically small and provide a relatively long operating life between battery recharging or replacement. The cost to a subscriber for a portable is expected, through very large scale integrated (VLSI) circuit implementations, to reside in the range of $100.00 to $350.00. In addition, each port would require a relatively small electronic package and carry an overall expected cost of less than $25,000.00--which is far less, by at least an order of magnitude, than that of a current cellular base station. Moreover, the digital data carried on each channel could be readily encrypted to provide a desired degree of security and privacy against eavesdropping. Furthermore, with this technology, a port antenna, due to its small size, could be readily moved within a building to cope with signal attenuation occurring therein. Port spacings would be properly established within the building and frequency re-use would be properly controlled between these ports to provide portable service having an acceptably low level of co-channel interference to a high density of users situated therein.
From a network perspective, low power portable digital telephony is extremely attractive. At present, approximately $50-100 Billion is invested by local operating telephone companies in costs associated with copper subscriber loops that run from distribution points to local telephone company demarcation points on individual customer drops. For a local telephone company, the per-subscriber cost of installing and maintaining a subscriber loop is generally greater at the loop end closest to a subscriber than at the far end thereof since the loop end is more dedicated to that subscriber than the far end is. Given the range provided by portable low power telephony, ports can be appropriately positioned throughout an area to provide radio link based exchange access and thereby substitute inexpensive mass produced VLSI circuitry for costly dedicated copper loops that would otherwise emanate from a distribution facility to an individual subscriber. Hence, by installing various ports throughout for example a building, significant labor intensive installation and maintenance tasks associated with re-wiring of telephone drops and re-location of telephone equipment would be eliminated with substantial savings being advantageously realized in attendant subscriber costs as people are moved from office to office therein.
Now, with the attractiveness of low power portable digital telephony being readily apparent, its success, in great measure, hinges on achieving satisfactory performance through the use of TDMA. TDMA, as currently envisioned for use in low power portable digital telephony, will utilize time multiplexed 164-bit bursts for communication from each of the portables to an associated port and 180-bit TDM packets for communication from that port to each of these portables. To yield a data rate of 16 kbits/second, two successive TDM/TDMA time slots are assigned by the port to each portable in use. Each TDM packet that is transmitted by the port in any one TDM time slot contains 180 bits. Of these bits, the first sixteen bits contain a pre-defined framing synchronization pattern, the next three bits are dummy bits, followed by 161 bits in which the first 147 bits contained therein hold data and the last 14 bits hold a parity sequence. Unfortunately, different propagation delays between the port and its associated portables and timing differences, the latter resulting from clock jitter occurring between the port and these portables, will both occur. Hence, to prevent different TDMA bursts that are transmitted from different portables from overlapping in time, a guard time having a 16 bit duration is used in lieu of the frame synchronization pattern in each TDMA burst transmitted by a portable to the port. The transmitter in the portable remains off during this guard time. Accordingly, each TDM packet transmitted from the port to a portable contains 180 bits with a self-contained synchronization pattern; while each TDMA burst transmitted from a portable to the port contains only 164 bits and no synchronization pattern.
Although TDMA has been successfully used for quite some time in fixed microwave satellite communications, the use of TDMA in the art of low power portable digital telephony is quite new. In general, the art has traditionally shunned the use of TDMA in such single user applications for a variety of reasons, one of which being the complexity inherent in controlling a TDMA channel.
In this regard, one crucial function required in TDMA for use in low power telephony is the need to achieve synchronization between a port and its associated portables. In particular, three levels of synchronization are needed: frame, burst and symbol synchronization. Frame synchronization is necessary for a portable to determine the start of a frame and the occurrence of its currently assigned TDM/TDMA channel therein. Frame synchronization is achieved in a portable by having the port continuously transmit in a TDM mode during which a known framing sequence, including "idle" information in idle TDM channels, would be transmitted at a known time relative to the start of each frame. The portable would extract frame timing of its associated port by using a digital correlator to reset a frame counter whenever the framing sequence was received. Once frame timing is determined, the portable would then expect to receive each of two individual successively occurring TDM packets arriving within a designated time window that is sufficiently wide to allow for slippage due to frequency drift. Symbol synchronization is needed to determine the start of a data symbol situated within any transmitted TDM packet or TDMA burst. Symbol synchronization can be achieved within either a port or portable by using a timing reference that is estimated from received data. As such, no need exists to transmit any special symbol synchronizing signal. Burst synchronization is necessary to ensure that a portable is able to discern when it should transmit a TDMA burst in response to a TDM packet received from the port and that both the port and portable are able to discern which specific bit in each received TDMA burst or TDM packet, respectively, is the first bit therein. Unfortunately, each TDM packet transmitted from a port to an associated portable is subject to timing misalignment. In the portable, this misalignment can arise from frequency drift. Burst misalignment can arise in a port from the different propagation time delays associated with the portables transmitting to that port, frequency drift of local oscillators and reference oscillator error in a port receiver. Burst misalignment can be large after an outage. Due to the need to detect and properly compensate for bit slippage arising from misaligned bursts or packets, burst and packet synchronization is difficult and complex to achieve and maintain over a sufficiently wide range of slippage.
Traditionally, the art teaches that synchronization and error detection should be handled separately using circuitry specifically designed for each task. Although this approach is flexible, it tends to waste bits and is complex.
Conventionally, synchronization is commonly performed by, for example, appending a known bit pattern, such as a "Barker" code, to each burst before its transmission. See, specifically R. A. Scholtz, "Frame Synchronization Techniques", IEEE Transactions on Communications, Vol. COM-28, No. 8, August 1980, pages 1204-1213; and generally with respect to achieving synchronization through use of CRC codes W. W. Peterson et al, Error-Correcting Codes (Second Edition) (.COPYRGT.1972, The MIT Press, Cambridge, Mass.--hereinafter referred to as the Peterson et al textbook) and specifically Chapter 12 "Synchronization of Block Codes" on pages 374-391 thereof, and J. J. Stiffler, Theory of Synchronous Communication, (.COPYRGT.1971, Prentice Hall Pub. Co., Englewood Cliffs, N.J.--hereinafter referred to as the Stiffler textbook) and specifically Chapter 14 "Synchronizable Error-Correcting Codes" on pages 453-511 thereof. This pattern would be detected, typically through correlation, in a receiver which would then synchronize its operation thereto. To provide a sufficiently low probability of acquiring false synchronization (commonly and hereinafter referred to as "falsing"), the known bit pattern itself would typically need to be at least 11 bits long with additional bits being required in the burst or packet for error control coding. Unfortunately using sequences of this length is only efficient with rather large bursts or packets, typically in excess of 1000 bits. With continuous rather than burst transmission, re-synchronization only needs to occur over relatively long bit times and on-going synchronization overhead can be substantially reduced, such as by periodically "stealing" one data bit, such as occurs with T1 channels, to maintain synchronization during these bit times.
Unfortunately, this conventional approach to achieving synchronization is simply not suited for use with low power digital portable telephony. In particular, low power digital portable telephony does not use continuous transmission from portables to a port but rather bursts that each contains fewer than 200 bits. In addition, synchronization must occur quite often: every time a new call is placed or handoff occurs; and for robustness, every time a portable comes out of a "fading" swell. As such, with the conventional approaches, a synchronization pattern would need to be added to each burst whenever re-synchronization was needed. Since the need could arise at any time, such a pattern would need to be added to every burst. While the sixteen bits transmitted from the port to a portable in each 180 bit burst contain a sixteen bit synchronization pattern; each 164 bit burst transmitted from the portable to the port contains no such pattern. Instead, as noted above, the sixteen bits are used not as conventional synchronization bits but rather as a guard time to accommodate burst misalignment due to timing and clock jitter, and propagation delays, and to accommodate as much as a twelve bit delay time required both by the portable to turn off its transmitter and for the transmitter in the portable using the next successive demand-assigned TDMA channel to turn on. As such, if a sixteen bit synchronization pattern were to be embedded within each 164 bit burst, then this would severely decrease the available informational bit rate between a portable and the port and hence unduly limit the amount of data that could be transmitted from each portable to the port. Accordingly, using the conventional approach would simply not be acceptable here.
Apart from synchronization, a second crucial function needed in TDMA is error detection: the ability of both a port and a portable to reliably determine whether the bit stream in any received TDM packet or TDMA burst contains an erroneous bit. Bits are frequently corrupted through interference and/or noise. If an erroneous bit occurs, then the packet or burst containing that bit needs to be blanked.
Error detection is conventionally achieved by adding one or more parity bits to each data word to form a codeword. In its simplest form, parity takes the form of one bit that represents odd/even parity. In sophisticated forms, multiple parity bits are used to store a cyclical redundancy code (CRC). Certain CRC codes have the desirable property that if only one bit in a data word is changed, then the code would change dramatically such that two simple compensating bit errors in the data word could not turn one codeword into another. This advantageously allows a receiver to detect the occurrence of an error condition in which a certain number of bits occurring randomly throughout the data word erroneously and simultaneously change. If additional bit errors occurred, then through use of a CRC code, the existence of these errors could still be detected with a given probability related to two exponentiated by the number of parity bits that form the CRC code. In this regard, see section 8.8 "Error Detection with Cyclic Codes" on pages 228-230 of the Peterson et al textbook.
It has been known in the art for some time that CRC codes could be used in the absence of bit errors to perform synchronization occurring up to a slippage of one less than half of the number of parity bits. This would be accomplished by inverting the first and last bit in the CRC code thereby forming so-called "marker bits". See, e.g., section 12.1 "Codes That Recover Synchronization Only" on pages 376-381 of the Peterson et al textbook. While, at first blush, use of such a CRC code to achieve synchronization would seem useful since the port and portable transmit TDM packets or TDMA bursts that, through slippage, are related in time. Unfortunately using such a CRC code is problematical. First, incorporating such a CRC only for synchronization into the 164-bit burst transmitted by each portable would reduce the informational bit rate that could be carried within each transmitted burst from a portable to the port and thereby wastes spectrum. Second and most important, use of such a code to extract synchronization requires that a nearly, if not completely, error free string of bits be received for each burst to ensure that falsing will not occur. As such, a TDMA channel that carries this burst must be error-free. Such a condition can not be satisfied. Accordingly, using such a CRC code only to recover synchronization is unsuitable for TDMA burst processing in low power portable digital telephony because of the high falsing rate that would likely occur.
Moreover, a number of schemes for performing combined synchronization and error detection/error correction has been developed in the art, as are also specifically discussed in the above-cited chapter 12, specifically section 12.3 therein entitled "Codes that Recover Synchronization and Correct Additive Errors" on pages 386-390 thereof, and chapter 14 respectively of the Peterson et al and Stiffler textbooks. However, each of these schemes requires the inclusion of many more bits in a codeword than would otherwise be needed for separate error correction and error detection procedures and thus also tends to waste spectrum. Any reduction in the number of these bits would disadvantageously reduce the amount of bit slippage over which synchronization could be achieved and/or the number of bits that could be corrected.
With these teachings in mind and given the importance of performing re-synchronization and error detection for each transmitted burst through error-prone channels in low power digital telephony and the limitations inherent in the approaches taught in the art, the art appears to conclude that two separate bit patterns should be included within each TDMA burst, one pattern being pre-defined for synchronization and the other being a CRC for error detection, even though this approach wastes spectrum. The importance of performing these tasks separately apparently outweighs a concomitant reduction in transmission bit rate from a portable to the port as well as implementation complexity and added cost for each portable.
Thus, a specific need exists in the art for a relatively simple technique for jointly and inexpensively performing bit synchronization and error detection on a TDMA burst of received digital data. Such a technique should utilize fewer bits than that used heretofore in the art and conserve available transmission spectrum. As such, such a technique would advantageously find use both in facilitating the use of TDMA and in assuring that TDMA will provide very acceptable performance within a relatively inexpensive low power portable digital radio communication system.